Hydrotalcite is a known natural mineral which is produced in relatively small amounts in limited areas. Hydrotalcite is known as an antacid which has a neutralizing effect on gastric juices. It is also known to produce synthetic hydrotalcites by the reaction of a carbonate source, a magnesium source and an aluminum source. U.S. Pat. Nos. 3,539,306 and 3,650,704 to Kumura et al discloses synthetic hydrotalcites and methods for manufacture of synthetic hydrotalcites. The synthetic hydrotalcite is described as having the following composition: EQU 6MgO.Al.sub.2 O.sub.3.CO.sub.2.12H.sub.2 O
According to these patents, the hydrotalcite is produced by the reaction of an aluminum component, which can be a water soluble aluminate, a magnesium component, which can be magnesium oxide or magnesium hydroxide, and a carbonate component, such as an alkali or alkaline earth bicarbonate in an aqueous medium at a temperature of 0.degree.-150.degree. C. at a pH of at least 8. These patents also discuss prior art procedures for preparing synthetic hydrotalcites which include the use of dry ice or ammonium carbonate as the carbonate source and using high temperatures and/or elevated pressures.
Hydrotalcites which have needlelike crystalline structures are described in U.S. Pat. No. 4,351,814 to Miyata. These hydrotalcites being prepared by reaction of a basic magnesium compound in the form of needlelike crystals with an aluminum cat ion under conditions which do not cause a loss of a needlelike crystalline shape of the basic magnesium compound. The reaction is carried out under a reaction temperature of about 90.degree. to 150.degree. C. These fibrous crystals are said to be useful as flame retardants for thermoplastic and thermosetting resins.
U.S. Pat. No. 4,629,626 to Miyata et al discloses a hydrothermally treated product useful for treating an iron deficiency, which compound has a hydrotalcite like crystal structure and includes iron, magnesium aluminum, and water within the structure. This product is prepared by an iron source, such as ferrous chloride with aluminum chloride and sodium hydroxides.
U.S. Pat. No. 4,883,533 to Kosin et al discloses a method for the manufacture of hydrotalcite and a modified synthetic hydrotalcite so produced. The preparation of the initial synthetic hydrotalcite takes place in an aqueous system which includes a source of magnesium, a source of aluminum and a source of carbonate. The magnesium source may be magnesium oxide, magnesium hydroxide, magnesium carbonate and other water soluble magnesium salts. The aluminum may be present as an aluminum salt including aluminum hydroxide, aluminum carbonate, aluminum alcoholates and other water soluble aluminum salts such as alkaline metal aluminates with the preferred aluminum source being sodium aluminate or as aqueous solution of solid alumina trihydrate. The carbonate reactant includes earth metal bicarbonates and alkaline metal bicarbonates with sodium bicarbonate as the preferred reactant. These reactants are reacted in a hydrothermal condition at 150.degree.-200.degree. C. for one to three hours to produce a hydrotalcite having average particle size of about one micron with a platelet morphology. When sodium aluminate is used as the aluminum source, the hydrotalcite is approximately 98 percent purity by x-ray diffraction analysis.
There remains a need in the art for an improved method of producing hydrotalcite having both a 99+% purity and small particle size using inexpensive, commercially available reactants.